Baby seats are well-known children's appliances and are already commercially available. Usually a safety guard and/or a tray are provided in front of the baby seat in order to prevent the baby from falling out and/or to provide a space for placing food/drinks. In addition, to facilitate accessing the baby seat the safety guard and/or the tray are designed in a removable type. For instance, U.S. Pat. No. 5,004,253 has disclosed a removable safety guard for a baby carriage which is removably coupled to the frame of the baby carriage through a coupling means (as the structure illustrated in FIG. 3 of this patent); the US patent application publication No. 2002/0175498 A1 has disclosed a partially removable front guard of a stroller which is connected to the frame of the stroller by way of a connecting member (as the structure illustrated in FIG. 2 of this patent application); and the US patent application publication No. 2002/0175498 A1 also disclosed a partially removable front guard of a baby seat. Although removable safety guards for a baby seat/stroller already exist, the connecting members between these existing safety guards and the baby seats/strollers possess a drawback of lacking sufficient mechanical strength which causes a problem that the connecting member will snap and fall out of the safety guard or the frame and unable to be used. In addition, some of the existing removable safety guard designs require both hands to operate and this will cause great inconvenience in a situation that one hand of the user is occupied (for instance, one hand of the user is holding a baby and thus only one hand is left to operate the removable safety guard). Therefore, there exists a need for a safety guard mounting/dismounting device for a baby seat which has sufficient mechanical strength and easy to operate.